Control surface stability with multiple vectors.

[ziomatrixacs]

Hey guys, Im trying to make a what I think would be the best precision aerobatic plane possible.. A plane where each axis (pitch roll and yaw) works independently without inducing any coupling.

I did a thought experiment particularly on the wing shape of most full scale aerobatic planes. They are almost stereotypically swept back LE and Swept forward TE with the aileron at an angle. And my conclusion was this question. If the Aileron is swept forward (that is, the tip is further forward on the chord than the root of the aileron) Wouldnt that make the air go both up and follow the aileron sweep? That would induce multiple vectors with the air flow correct? Yet these planes obviously fly aerobatics well enough. Im honestly not sure of a better way to describe it with words..so I am trying to illustrate my idea.

I know airplanes roll much better when the majority of the surface area of the aileron is furthest from the axis of rotation (fuselage) But if the wings are pointed in such a way, could it be possible to make a model be super stable in a torque roll? Please refer to my drawings, Im sure they better present my idea than words.. :o

[vintage1]

intersting discussion: I got asked to design a really fast aerobatic plane that would also hover! some challenge. I'm still thinking about it..

One thing I think is that a teeny bit of sweepback will have beneficial yaw/roll coupling whichever way up the model is...

..another is that larger inboard ailerons are vital for roll axis control in the hover.

..a third is that a thinner section and a reasonably high aspect ratio is probably better for speed, but may show badly in violent manouvers, lowering roll rate and being more prone to snap stall..

So I am just tossing those in to what you have already posed to ponder about.

[ziomatrixacs]

Yeah, thank you for mentioning the inboard section of the aileron having to be large for hovering. I meant to mention it but completely forgot.

When you hover, you only have the air that the prop blows over the control surfaces for control...so the area at the tips basically becomes useless. In straight and level flight, the area at the tips provide all of the torque to make that frame roll fast.

To test this, I built 2 foam planes (thank the lord for foam in these type of planes!) One is a biplane similar to the AITO and the other is a mono wing called the Nikitis HOTROD. The bipe had a massive amount of aileron area near the fuse (around 4") and about 3/4th inch at the tip so there is considerable taper! The ailerons were straight, and not slanted. It had plenty of authority in hover (Note, I said authority, not straightness, the rolls were far from axial) But it had about a half turn per second roll rate in level flight. Plus it felt sluggish. I then added the foam I cut off to make the ailerons rectangles and the planes performance increased greatly. Rolls were very consistent in both straight , vertical and hover not to mention adding about 40 -60 square inchs of wing area so it flew noticeably slower. One thing I did note especially was that it could do knife edge loops with little coupling but it didnt seem to change much before and after the aileron change. So if these guys flying that $100 aito added a triangle to the ailerons, they might end up with a plane that flies better..thats just an idea though.
The hotrod is similar to a flat foam edge 540, with the tapering wings..It is the best foam plane I have ever flown IMO. It has a bit of coupling in K. edge loops but its torque rolls are very axial (but still not perfect), so are its vertical and level rolls. They are also very consistent in speed.

So I find irony in this, In order to get consistent rolls, it helps to have the ailerons swept forward, and yet in order to try to derive a "pure" aerobatic precision plane, you want each axis to function independently. So it seems to work like these fireworks.
http://img.alibaba.com/photo/10248038/Fireworks_Flying_Dragon.jpg
They fly by having one or two jets spaces at a 90 degree angle (and opposite of course to produce a torque to spin the body of the firework. The blades turn with the firework body and it flies.. So if the ailerons push air in the same way the fireworks jets produce a torque around the body, could the ailerons being angled actually increase the roll rate? At first thought it seems like it could increase the speed of the plane but I think it doesnt. At first thought, it would seem like the air gets pushed backwards and a little bit of that air increases the roll rate and another bit increases the speed. This is because the air coming off the aileron is at an angle to the fuselage, and if you break that vector up into its X and Y components, You have one pushing parallel to the fuselage (assuming its not EVP!) and you have one pushing perpendicular to the fuselage and of course you have the Z component that you normally see when you think of an aileron rolling, the component that pushes the wing up and down. The parallel to the fuselage may increase the speed of the aircraft some how..but I feel that it may cause drag and any speed increase be due to you having to increase the power to compensate. (Since I doubt we can precisely compensate by looking at the plane fly and hit the throttle stick to get a 1-2mph speed increase in a plane moving 30+mph)


Another question I may pose is the aileron differential that is induced by the way most foamies hinge their control surfaces. Usually they use tape hinges where one side is beveled and the top is flat. So you get noticeably different movement on one end of travel. Not alot of people correct this...but I know this has a great effect on the way it rolls since it is just differential. Could this possibly be used to our advantage in making an aerobatic plane?


Ill admit, my English isnt the best and I have limited knowledge towards aerodynamics..but I sure as heck can spot a torque or force vector out when I see one! So if anyone can add on to the discussion, please correct me (my theories, not my English :) )

[MCarlton]

I got asked to design a really fast aerobatic plane that would also hover! some challenge.

This might sound daft, it probably is, but would variable geometry help here? Think of a reasonably thick wing, with the max thickness line at 90 degrees to the fus datum. So taper on LE and TE.

If you sweep that wing back, does that reduce drag?

You could arrange the ailerons to be split spanwise, so that in "wing forward" mode, both surfaces move as full span ailerons, but activation of the "swept" position isolates the inner surfaces so they can slide inside the fus, but you keep the outer surfaces.

Just a thought

[ziomatrixacs]

It would reduce the frontal area so you get less drag that way..

Hmm when the wings are back, the CG is back as well. When the wings sweep forward, the CG moves forward slightly but it would still be back. IE you get a ton of pendulum stability just for hovering.
It sounds plausible..but you would need a metal gear servo to do the wing sweeping, and 4 ailerons servos plus 2 servos for the tail.. 7 servos to run the plane, you need a standard flight NiMh pack or something and set it up like the ultra stick.. The ultra stick has 4 ailerons, if you make them spread out opposite of each other, they become super effective spoilers and make the plane almost stop dead in the air.

IMHO you would be best off with a variable pitch prop from a helicopter tail rotor on a .46 -50 size nitro engine to pull that off... is there any restrictions to power source and speed?